The present invention relates to a tool set adapted to be mounted in and to be driven by a press so as to join or connect a plurality of sheet metal work pieces. In U.S. Pat. No. 3,771,216 a tool set is disclosed that produces such a connection. In this reference, a punch deforms a portion of two overlapped metal sheets so that the deformed portions of the metal fill a die cavity without cutting through the sheets. Connections produced by such a tool have several advantages: they are fluid-tight, have an attractive shape and appearance, and are relatively strong.
The tool set disclosed in the referenced document provides that the deformed material will completely fill the cavity defined by the anvil, die, and punch. There is no means disclosed to remove debris from the bottom of the cavity while the punch is in the cavity. However, work pieces to be joined frequently are oily or greasy or have other incompressible impurities, such as dust, metal, chips, iron filings, coatings, varnish, etc on their surface. Consequently, when the pressing force applied thereon is predetermined, the connection may be insufficiently strong and may fail. If the press has a predetermined stroke length, the unavoidable irregular thickness tolerances of the pieces of sheet metal can result either in a joint of insufficient strength or, on the contrary, in the development of extremely high expansive forces to be absorbed by the die which, consequently, must be very rigid and is therefore relatively cumbersome.